3 Answers
3

The answer is No! Omar was not an informant. Sosa killed Omar because he knew if he let Omar go back to Miami that him and Frank would never make the deal. He could tell that Omar knew Frank wouldn't be interested in a deal that big. But he knew Tony would be and that if he sent Tony back alone that the deal would most likely get done. If not, he would kill them just like he did Omar and as we all know later on Tony as well. Frank was just surprised and disgusted that Tony tried to make the deal himself and suddenly was so enamoured with Sosa who he already didn't care for. As you saw throughout the movie Sosa did away with pretty much anyone who stood in his way with making $ .

In addition to that this phone call during the meeting was likely nothing else than Sosa eavesdropping on what Tony and Omar were discussing about, substantiating Sosa's impressions about the deal even further.
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Tom Cody♦Aug 19 '14 at 23:56

I do not think that Omar was an informant. I think that Sosa sensed that Omar was not behind the plan for a big deal where Tony was. Therefore, he killed the source of resistance and sent Tony, who was all for the deal, back to Frank to try to make it happen.

It was similar to the scene in The Godfather where Sollotzo pitched the idea of the Corleone's getting into the drug business to Vito Corleone. During the meeting, Sonny Corleone showed his hand that he was excited by the idea, whereas Vito Corleone was not and refused to do business with Sollotzo. Sollotzo then tried to kill Vito Corleone, so that Sonny, who would become the Don, would agree to get into the drug business.

It is not shown, nor implied in the movie whether Omar really was an informant. However, given the situation, the logical answer to the question seems to me to be yes.

If Omar really was an informant, would Frank really have known about it? Of course not! Frank's denial or reply that Sosa was lying had more to do with his anger at that moment. Tony had just come back with an unauthorized deal with Sosa without Frank's nod and claimed that they killed Omar(who was Frank's man) because he was an informant. Obviously, Frank went in denial. So as a personal opinion, yes it looked that way. Omar was an informant.